Extruded member of aluminum alloy excelling in flexural crushing performance and corrosion resistance and method for production thereof

ABSTRACT

An extruded member of Al—Mg—Si aluminum alloy specially composed of Mg, Si, Fe, Cu, Zn, Ti, etc. which has the equiaxed re-crystallized grain structure in which intergranular precipitates 1 μm or lager are separate from one another at large average intervals and there are many cube orientations over the entire thickness region thereof so that it excels in both flexural crushing performance and corrosion resistance. The extruded member is suitable for use as automotive body reinforcement members which need outstanding lateral crushing performance under severe collision conditions as well as good corrosion resistance.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an extruded member of Al—Mg—Si aluminum alloy excelling in flexural crushing performance and corrosion resistance and a method for production thereof. (“Aluminum” may be referred to as “Al” for short hereinafter.) The term “extruded member of aluminum alloy” used in the present invention denotes not only any members produced by hot extrusion but also any parts incorporated into automotive bodies as their reinforcement members (or energy absorbing members) mentioned later.

2. Description of the Related Art

Extruded members of aluminum alloy of 6000 series have been used as reinforcement members. For their improvement in lateral crushing performance (deformation due to crushing in the direction of cross section) and bending formability, much has been suggested about their metallographic structure.

One of such suggestions is about the method for producing extruded members of aluminum alloy by soaking billets of aluminum alloy of 6000 series (such as 6063), extruding the billets, and cooling and ageing the extrudates. It is suggested that ageing should be so performed as to provide mechanical properties specified by 0.2% proof stress of 120-140 MPa and elongation of 12% or more. This method is intended to produce extruded members of aluminum alloy which have adequate 0.2% proof stress and elongation for bending, limited variation in bending accuracy and yield strength, and high resistance to buckling that occurs during bending by “push through”. (See Japanese Patent Laid-open No. 2001-316788.)

There is another suggestion about improvement in bending performance by causing the extruded member of aluminum alloy of 6000 series to have the equiaxed granular structure, as disclosed in Japanese Patent Laid-open No. 2002-241880. According to this literature, the object is achieved when the aluminum alloy contains Mg and Si in stoichiometrically equal amount and also contains such transition metal elements as Mn, Cr, and Zr (that promote the formation of fibrous structure) in a total amount of 0.1% or less and the extrusion temperature is 500° C. or above and extrusion is immediately followed by water quenching (forced cooling). The resulting equiaxed granular structure is such that the average grain size is 100 μm or smaller and the aspect ratio of crystal grain is no larger than 2. The aspect ratio is a length-to-thickness ratio of a crystal grain, with the length measured in the direction of extrusion.

There is further another suggestion disclosed in Japanese Patent Laid-open No. Hei-5-171328. This literature suggests that hollow extruded members improve in bending formability if they have the fibrous structure (with crystal grains elongated in the direction of extrusion) in place of the equiaxed granular structure mentioned above. According to this literature, the extruded member is produced from an aluminum alloy containing such transition metal elements as Mn, Cr, and Zr in a comparatively large total amount of 0.45-0.53% by extrusion at 500° C. or above, which is immediately followed by water quenching (forced cooling) in a water bath.

It is known that the fibrous structure mentioned above is effective for such extruded members as side member and bumper stay to be used as energy absorbing members which need good longitudinal crushing performance in their axial (or lengthwise) direction, so that they resist Euler buckling (bending in a dogleg shape) but undergo deformation in a bellow shape. See Japanese Patent Laid-open Nos. Hei-9-256096 and 2003-183757. The former literature proposes an extruded member of aluminum alloy which contains Mg and Si in a stoichiometrically equal amount so that it has the fibrous structure mentioned above. It also suggests that the tendency toward transformation into recrystallization structure due to Mg and Si contained in a stoichiometrically equal amount is avoided if the extruded member contains such transition metal elements as Mn, Cr, and Zr in a comparatively large total amount of 0.5% and extrusion is performed at 500° C. or above and immediately followed by water quenching.

Japanese Patent Laid-open No. 2003-183757 mentioned above proposes an extruded member of aluminum alloy of 6000 series which contains excess Si and also contains such transition metal elements as Mn, Cr, and Zr in a comparatively large total amount of 0.25-0.48%. According to this literature, extrusion is performed at 500° C. and the fibrous structure has a specific thickness of recrystallized layer (GG layer) and a specific grain size, so that the extruded member exhibits not only good longitudinal crushing performance but also good lateral crushing performance.

It is suggested in Japanese Patent Laid-open No. 2005-105317 that the extruded member of aluminum alloy of 6000 series to be used as reinforcement members should have not only fibrous structure but also anisotropically elongating structure so that it possesses both good bending formability and good crush-cracking resistance. According to this literature, the extruded member of aluminum alloy contains excess Si and also contains such transition metal elements as Mn, Cr, and Zr in a comparatively large total amount of 0.15-0.30%. Moreover, it mentions that extrusion should be performed at a comparative low temperature under 500° C. with a high extrusion ratio over 10, so that the extrudate has the fibrous structure composed of crystal grains elongating in the direction of extrusion, with the aspect ratio exceeding 5. The resulting extruded member has an anisotropic structure such that the elongation (δ1) in the direction deviating by 45 degrees from the direction of extrusion is larger than elongation (82) and (83) in the direction parallel and perpendicular, respectively, to the direction of extrusion.

It is also suggested in Japanese Patent Laid-open No. Hei-6-25783 that the extruded member of aluminum alloy of 6000 series to be used as side members and bumper reinforcement members should have the equiaxed grain structure (with the aspect ratio of crystal grains being no larger than 3) instead of the fibrous structure so that it has both good bending formability and good impact absorbing performance. [Aspect ratio is a ratio in length of the long axis to the short axis of a crystal grain.] According to this literature, the fine equiaxed grain structure contributes to improved elongation and bending formability and also restricts the amount and size of intergranular precipitate, thereby preventing fragmentation of crystal grains from occurring at intergranular precipitates at the time of impact.

In a practical situation where the extruded member of aluminum alloy of 6000 series is used as automotive reinforcement members, such as bumper reinforcement members and door guard bars, they usually receive a concentrated collision force in the approximately horizontal direction. In such a situation, the extruded member of aluminum alloy of 6000 series is poor in flexural crushing performance, which is important for improvement in lateral crushing performance, even though it has the fibrous structure or anisotropic structure (as suggested in Japanese Patent Laid-open Nos. Hei-5-171328, Hei-9-256096, 2003-183757, and 2005-15317) or it has the equiaxed grain structure (as suggested in Japanese Patent Laid-open Nos. 2003-241880 and Hei-6-25783).

Collision in the horizontal direction is typically pole collision and offset collision. In the case of such collision, the collision force in the horizontal direction locally concentrates on the automotive reinforcement member, such as bumper reinforcement member, thereby bending it in its lengthwise direction at the part of collision (which receives the load of collision) and causing damage to the automotive body.

To cope with collision under critical conditions, it is necessary to improve the extruded member of aluminum alloy of 6000 series in flexural crushing performance. Meeting this requirement is limited even with the comparatively strong extruded member having the fibrous structure mentioned above, as well as the extruded member having the equiaxed grain structure disclosed in the two prior art technologies mentioned above.

The present invention was completed in view of the foregoing. It is an object of the present invention to provide an extruded member of aluminum alloy of 6000 series and a method for production thereof, said extruded member having both good flexural crushing performance and good corrosion resistance which are required of reinforcement members of automotive body subject to collision under more critical conditions.

OBJECT AND SUMMARY OF THE INVENTION

The present invention to achieve the above-mentioned object is directed to an extruded member of aluminum alloy which contains (in mass %) Mg: 0.60-1.20%, Si: 0.30-0.95%, Fe: 0.01-0.40%, Mn: 0.001-0.35%, Cu: 0.001-0.65%, Zn: 0.001-0.25%, and Ti: 0.001-0.10%, with the remainder being aluminum and inevitable impurities, and has the metallographic structure whose cross section perpendicular to the direction of extrusion shows the equiaxed recrystallized grain structure in which intergranular precipitates 1 μm or lager in terms of the diameter of an equivalent circle are 3 μm or more separate from one another in the observation under a TEM of 5000 magnifications and also the average areal ratio of cube orientation is 15% or larger over the entire thickness region including the grain growth layer in the outermost surface in the cross section perpendicular to the direction of extrusion.

The extruded member of aluminum alloy should preferably contain Mg and Si such that

Mg(%)≧1.73×Si(%)−0.4

where Mg(%) and Si(%) denote the content of Mg and Si in mass %, respectively.

The extruded member of aluminum alloy mentioned above should preferably have the equiaxed recrystallized grain structure such that the average areal ratio of cube orientation is 20% or larger. Also, the extruded member of aluminum alloy mentioned above may selectively contain at least either of Cr: 0.001-0.18% or Zr: 0.001-0.18% in a total amount of 0.30% or less. The extruded member of aluminum alloy mentioned above should preferably have the flexural crushing performance such that the critical bending radius (R) is 3.0 mm or smaller which does not cause cracking in the 180° bending test according to JIS Z2248 in which the platy specimen is bent in the direction of extrusion, and the extruded member of aluminum alloy mentioned above should preferably have the corrosion resistance such that the specimen does not suffer intergranular corrosion in the alternating immersion corrosion test according to ISO/DIS 11846B.

The extruded member of aluminum alloy mentioned above will find use as energy absorbing members which crush under load in the direction perpendicular to the direction of extrusion.

The present invention to achieve the above-mentioned object is directed to a method for producing an extruded member of aluminum alloy, said method comprising a step of soaking a cast billet of aluminum alloy at 500-590° C., said billet containing (in mass %) Mg: 0.60-1.20%, Si: 0.30-0.95%, Fe: 0.01-0.40%, Mn: 0.001-0.35%, Cu: 0.001-0.65%, Zn: 0.001-0.25%, and Ti: 0.001-0.10%, with the remainder being aluminum and inevitable impurities, a step of subjecting the soaked billet to forced cooling to 400° C. or below at an average cooling rate of 100° C./hr or above, a step of reheating the cooled billet and subjecting the reheated billet to hot extrusion such that the extrudate reaches the solid solution temperature which is 500° C. or higher at the extruder exit, a step of immediately subjecting the extrudate to forced cooling at an average cooling rate of 100° C./hr or above, and a step of subjecting the cooled extrudate to ageing, so that the resulting extruded member has a 0.2% proof stress of 240 MPa or greater and also has the metallographic structure whose cross section perpendicular to the direction of extrusion shows the equiaxed recrystallized grain structure in which intergranular precipitates 1 μm or lager in terms of the diameter of an equivalent circle are 3 μm or more separate from one another in the observation under a TEM of 5000 magnifications and also the average areal ratio of the cube orientation is 15% or larger over the entire thickness region including the grain growth layer on the outermost surface in the cross section perpendicular to the direction of extrusion.

The cast billet of Al—Mg—Si aluminum alloy mentioned above may selectively contain at least either of Cr: 0.001-0.18% or Zr: 0.001-0.18% in a total amount of 0.30% or less.

The present inventors paid attention to the texture of the extruded member of aluminum alloy of 6000 series which had not attracted attention so much in the past, and they investigated anew the effect of the texture on the flexural crushing performance and corrosion resistance. As the result, they found that the texture, particularly the equiaxed recrystallized grain structure having the cube orientation, effectively improves flexural crushing performance and corrosion resistance.

Much has been studied about the texture of aluminum alloy of 6000 series in the field of rolled plate to elucidate its effect on the press formability and bending formability of automotive panels. (Bending includes hemming, particularly flat hemming.) There are numerous prior art technologies based on such studies. A typical one of them teaches that the texture of aluminum alloy of 6000 series is more effective in improvement of flat hemming performance according as the crystal grains having the cube orientation increases in its ratio. As known well, the cube orientation is the major orientation of the texture in the rolled sheet of aluminum alloy of 6000 series. It is also one of the major crystal orientations in Al—Mg—Si alloys.

Unlike extruded members (for use as reinforcement members), the rolled sheets of aluminum alloy of 6000 series are used as automotive body panels and hence they are very thin (about 1 mm or below) for weight saving. Moreover, when they undergo press forming and bending, they receive bending load which is different from collision load the extruded members receive. The bending load is applied by molds and punches almost uniformly over a broad area of the sheet. In addition, the rolled sheet has a comparatively low strength (150 MPa or lower in terms of 0.2% proof stress), even in the case of T4 material, in consideration of formability for automotive body panel.

The present invention, however, is intended for extruded member for reinforcement which has a comparatively great wall thickness of 2 mm or thicker and also has a rectangular hollow cross section. This extruded member is a high-strength one having 0.2% proof stress of 240 MPa or higher. The above-mentioned rolled sheet receives a bending load when it undergoes hemming, but the bending load basically differs in deformation mechanism and pattern from that which the extruded member according to the present invention experiences at the time of vehicle collision (such as pole collision or offset collision) involving locally concentrated loads. The relation between flat hem formability and cube orientation in the texture of rolled thin sheet of aluminum alloy of 6000 series is useless for predicting how corrosion resistance and flexural crushing performance are related with cube orientation in the texture of the extruded member of aluminum alloy of 6000 series according to the present invention.

In the field of extruded member of aluminum alloy of 6000 series, it has been common practice to cause the extruded member to have the fibrous structure elongating in the direction of extrusion in order that the resulting hollow extruded member has good crushing performance in the lengthwise (or axial) direction and lateral (or crosswise) direction, as disclosed in Japanese Patent Laid-open No. Hei-5-171328 mentioned above. Such fibrous structure has the texture in which cube orientation does not develop and the ratio of cube orientation (or crystal grains having cube orientation) is limited to a very small value. The common knowledge in the field of extruded member of aluminum alloy of 6000 series does not permit one to predict how flexural bending performance and corrosion resistance are related with cube orientation in the texture of the extruded member of aluminum alloy of 6000 series according to the present invention. The foregoing is the reason why the texture of extruded members has not attracted attention so much although there are some reports about whether the extruded member of aluminum alloy of 6000 series should have the fibrous structure or the equiaxed grain structure.

According to the present invention, the extruded member of aluminum alloy of 6000 series is designed to have the texture in the form of equiaxed recrystallized grain structure with increased cube orientation so that it is improved in flexural crushing performance and corrosion resistance. Thus, the extruded member of aluminum alloy of 6000 series can be used as energy absorbing members, such as bumper reinforcement and door guard bar, which crush under loads in the lateral direction, in the same way as the extruded member of aluminum alloy of 7000 series which has a comparatively high strength, with the former outperforming the latter in corrosion resistance.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following is a detailed description of the extruded member of aluminum alloy of 6000 series according to the present invention.

(Texture)

As mentioned above, the extruded member of aluminum alloy of 6000 series as a reinforcement member improves in flexural crushing performance according as the ratio of crystal grains having cube orientation increases in its texture. It also improves in resistance to such corrosion as intergranular corrosion in corrosive environment like saline, according as the ratio of crystal grains having cube orientation increases.

The present invention should meet the following requirements in order that the extruded member used as reinforcement members has improved flexural crushing performance and corrosion resistance. The extruded member should have the metallographic structure whose cross section in the thickness direction shows the equiaxed recrystallized grain structure in which intergranular precipitates 1 μm or lager in terms of the diameter of an equivalent circle are 3 μm or more separate from one another in the observation under a TEM of 5000 magnifications and also the average areal ratio of the cube orientation is 15% or larger, preferably 20% or lager, over the entire thickness region including the grain growth layer in the outermost surface in the cross section in the thickness direction.

The present invention is designed such that the areal ratio of cube orientation is made large regardless of the amount (or areal ratio) of other orientations such as Goss orientation. If the average areal ratio of cube orientation is too small, there are too few crystal grains having cube orientation and hence the resulting extruded member does not improve in flexural crushing performance and corrosion resistance and hence it does not meet requirements (specifications) for use as automotive reinforcement members.

(Relation of Cube Orientation to Flexural Crushing Performance and Corrosion Resistance)

According as the ratio (or number) of crystal grains having one orientation such as cube orientation increases, the difference in orientation of the grain boundary of crystal grains decreases. Thus, concentration of stress to the grain boundary is released when flexural load is applied (or collision load is applied). As the result, particularly in the case of energy absorbing members such as bumper reinforcement and door guard bar, the flexural crushing performance improves at the time of crushing (lateral crushing) due to external load (flexural load) such as collision in the direction perpendicular to the direction of extrusion of the extruded member of aluminum alloy constituting it.

Also, crystal grains having cube orientation possess the characteristics that crystals hardly rotate when they receive deformation such as tensile and flexure due to the above-mentioned load in the direction of extrusion (lengthwise direction) or the widthwise direction (direction perpendicular to the direction of extrusion). Consequently, if cube orientation develops, the ratio of crystal grains having cube orientation becomes large and the difference in orientation of grain boundary of crystal grains becomes small, this small difference in crystal orientation is maintained even after large deformation of tensile and flexure due to said load is received. Owing to such an inherent effect of cube orientation, stress concentration to grain boundary that occurs when flexural load is applied (or collision load is applied) is released and the flexural crushing performance improves.

On the other hand, if cube orientation does not improve, even though other orientation, typically such as Goss orientation, develops, particularly in the case of deformation in the direction of extrusion (lengthwise direction) or widthwise direction (perpendicular to the direction of extrusion), the above-mentioned characteristics inherent to cube orientation do not exist (are not exhibited). Therefore, in the case of deformation due to flexural load, the flexural crushing performance decreases. The phenomenon that stresses concentration to the grain boundary increases when flexural load is applied and the flexural crushing performance decreases is the same as that in the case of the above-mentioned fibrous structure having a comparatively high strength. This is the reason why the flexural crushing performance of the conventional fibrous structure is largely limited.

And, according as cube orientation develops and the difference in orientation of the grain boundary of crystal grains becomes smaller, corrosion resistance such as grain boundary corrosion resistance improves. On the other hand, if cube orientation does not develop and the difference in orientation of the grain boundary of crystal grains becomes larger, corrosion resistance such as grain boundary corrosion resistance tends to decrease.

(Reason for Defining All Areas in Thickness Direction of Cross Section of Extruded Member)

In the present invention, cube orientation is defined as the average areal ratio over the entire thickness region including the outermost grain growth layer in the cross section in the thickness direction (or the cross section in the direction of extrusion and in the direction perpendicular to the direction of extrusion) of the extruded member. In the cross section in the thickness direction of the extruded member, there exist usually on both sides of the outermost surface the grain growth layer (GG layer or layer of coarse recrystallized grain structure) with a thickness of several hundred microns which inevitably occurs as the outermost surface comes into contact with the extrusion die. In the GG layer in the outermost surface, random orientations predominate, cube orientation does not develop, and crystal grains having cube orientation are very few. Therefore, the thicker the GG layer in the outermost surface, the thinner the equiaxed recrystallized grain structure, which is inside in the thickness direction of the extruded member and in which cube orientation develops, and the effect of improving flexural crushing performance becomes smaller. In other words, the degree of development of cube orientation over the entire region in the thickness direction in the cross section of the extruded member, or the ratio of crystal grains having cube orientation, determines the flexural crushing performance of the extruded member used as reinforcement member. Therefore, in the present invention, particularly, in order to improve the flexural crushing performance as reinforcement member, cube orientation is prescribed in terms of the average areal ratio over the entire region of the thickness of the extruded member, including the outermost grain growth layer in the cross section in the thickness direction.

Also, with the help of the crystal orientation analyzing method (SEM/EBSP method) mentioned later, it is possible to measure cube orientation over the entire thickness region including the outermost grain growth layer, for example, over the region broader than a thickness of 2 mm of the extruded member, and it is also possible to obtain the average of the areal ratio. By contrast, X-ray diffraction (or X-ray diffraction intensity), which is commonly used for measurement of the texture, is designed to measure the structure (or texture) in a comparatively micro region for each crystal grain as compared with the crystal orientation analyzing method that employs SEM/EBSP. Therefore, the X-ray diffraction method needs a large number of measurements to cover the area larger than 2 mm over the entire region in the thickness direction of the extruded member, and it is practically incapable of measuring the average areal ratio of cube orientation over the entire region in the thickness of the extruded members as defined in the present invention.

(Equiaxed Recrystallized Grain Structure)

The reason why the present invention is intended for the extruded member to have the equiaxed recrystallized grain structure is that the fibrous structure as disclosed in Japanese Patent Laid-open Nos. Hei-5-171328, Hei-9-256096, 2003-183757, and 2005-105317, in which the crystal grain has an aspect ratio exceeding 5 and the crystal grain elongates in the direction of extrusion, does not permit cube orientation to develop to such an extent that the average areal ratio of cube orientation over the entire region of the cross section of the extruded member in the thickness direction is 15% or higher. The term “equiaxed recrystallized grain structure” denotes the equiaxed grain structure in which crystal grains have an average aspect ratio of 3 or smaller and the average aspect ratio is lower than 5 even though crystal grains elongate in the direction of extrusion. The term “aspect ratio of crystal grain” means the ratio of the long axis to the short axis, with the long axis being measured in the direction of extrusion and the short axis, in the thickness direction.

The prior art technologies disclosed in Japanese Patent Laid-open Nos. 2002-241880 and Hei-6-25783 mentioned above are intended for the extruded member of aluminum alloy of 6000 series to have the equiaxed recrystallized grain structure. However, they are able to provide the equiaxed recrystallized grain structure but unable to make cube orientation develop to such an extent that the average areal ratio of cube orientation over the entire region of the cross section of the extruded member in the thickness direction is 15% or higher. The prior art technology disclosed in Japanese Patent Laid-open No. 2002-241880 is designed to produce the extruded member in such a way that the content of Mg and Si is stoichiometrically equivalent so that the equiaxed grain structure develops and the total amount of transition metal elements, such as Mn, Cr, and Zr, that promote the formation of fibrous structure, is limited to 0.1% or less and extrusion is performed at 500° C. or above and extrusion is immediately followed by water quenching for forced cooling. The extruded member produced in this manner has the equiaxed grain structure in which the average crystal grain size is 100 μm or smaller and the aspect ratio of the crystal grain (the ratio of the length of the crystal grain in the direction of extrusion to the length of the crystal grain in the thickness direction) is 2 or smaller. Japanese Patent Laid-open No. Hei-6-25783 discloses in its Example an extruded member of aluminum alloy of 6000 series which has the structure of excess Si type and optionally contains transition metal elements, such as Mn, Cr, and Zr, in a comparatively large total amount of 0.34%. The extruded member does not undergo forced cooling such as water quenching (that immediately follows extrusion) on-line but undergoes separately solid solution treatment and quench hardening off-line.

By contrast, in order for the extruded member to have the grain structure such that cube orientation develops to such an extent that its areal ratio exceeds 15% over the entire region of the cross section in the thickness direction, as intended in the present invention, it is necessary to positively increase the areal ratio of cube orientation by controlling the manufacturing condition, such as forced cooling that follows soaking treatment, as mentioned later. Also, the composition of the extruded member disclosed in Japanese Patent Laid-open No. Hei-6-25783, which is not of extremely excess Si type and contains transition metal elements, such as Mn, Cr, and Zr, in a reduced amount, is one condition for cube orientation to develop, as mentioned later. Therefore, the extruded members disclosed in Japanese Patent Laid-open Nos. 2002-241880 and Hei-6-25783, which were produced by the process merely involving ordinary soaking treatment without positive control mentioned above, do not permit cube orientation to develop even though they are extruded under the same condition. In other words, the extruded members according to the prior art technologies have the texture with random crystal orientations and hence they have an average areal ratio of cube orientation which is inevitably smaller than that in the extruded member according to the present invention. That is, the ordinary manufacturing method gives extruded members which have the equiaxed grain structure but do not have the equiaxed grain structure in which cube orientation develops as intended in the present invention.

(Measurement of Cube Orientation)

The areal ratio (or the existence ratio) of the orientation of each crystal grain (orientation components of each crystal grain), including cube orientation, is measured by means of the crystal orientation analyzing method (SEM/EBSP method) that employs EBSP (electron backscatter diffraction pattern) with help of an SEM (scanning electron microscope).

The crystal orientation analyzing method that employs EBSP is carried out in such a way that a specimen placed in the lens barrel of an SEM is irradiated with electron beams so that an EBSP is projected onto a screen. The image on the screen is photographed by a high-sensitivity camera and taken into a computer. In the computer, the image is analyzed and compared with the pattern which has been obtained by simulation with a known crystal, so that the crystal orientation is identified.

The crystal orientation analysis with EBSP is not performed on individual crystals but is performed on a specified region of specimens by scanning at certain intervals. Therefore, the above-mentioned process is performed on all the points of measurement automatically, and hence there are obtained tens to hundreds of thousands of data for crystal orientation at the end of measurement. This method of measurement offers the advantage of permitting observation over a broad field of view and providing information about a large number of crystal grains, including average crystal grain size, standard deviation of average crystal grain size, and orientation analysis, within a few hours. Therefore, it is most suitable for the extruded member according to the present invention for which the texture is to be analyzed over the entire region in the thickness direction or a broad area of 2 mm or thicker in thickness, including the GG layer in the outermost surface.

The crystal orientation analysis with EBSP uses a specimen for observation of structure which is taken from the cross section of the extruded member. The cross section covers all the directions in the thickness, including the outermost GG layer. The specimen is prepared by mechanical polishing, buffing, and electrolytic polishing. The resulting specimen is examined by, for example, JEOLJSM 5410 (SEM from Nippon Denshi) or EBSP measuring and analyzing system “OIM” (Orientation Imaging Macrogroaph), bundled with an analyzing program called OIMAnalysis, from TSL. The analysis judges whether or not each crystal grain has the desired orientation (or within 15° from the ideal orientation) and then determines the density of orientations in the field of view for which measurement has been carried out. Measurement is carried out at several points 3 μm or less apart in the cross section of the extruded member, and the values of measurements are averaged.

The region of specimen for measurement is usually divided into hexagonal sections and each section is irradiated with electron beams so that reflected beams form the Kikuchi pattern. Two-dimensional scanning with electron beams to measure the crystal orientation at prescribed intervals gives the distribution of orientations of the specimen. The thus obtained Kikuchi pattern is analyzed to define the crystal orientation at the position for incident beams. In other words, the resulting Kikuchi pattern is compared with the data of a known crystal structure to identify the crystal orientation at the measurement point.

(Texture)

Incidentally, as mentioned above, the texture including the cube orientation of the extruded member is examined in the same way as for rolled sheets, with the measurement point being regarded as a plate.

Each orientation is represented as follows according to “Texture” compiled by S. Nagashima (published by Maruzen)) and “Light Metals” compiled by Institute of Light Metals, vol. 43 (1993), pp. 285-293.

-   Cube orientation: {001}<100> -   Goss orientation: {011}<100> -   CR orientation: {001}<520> -   RW orientation: {001}<110> [corresponding to Cube orientation turned     with respect to the (100) plane] -   Brass orientation: {011}<211> -   S orientation: {123}<634> -   Cu orientation: {112}<111> (or D orientation: {4411}<11118> -   SB orientation: {681}<112>

(Intergranular Precipitates)

In order for the extruded member of aluminum alloy of 6000 series to exhibit good flexural crushing performance and corrosion resistance when used as reinforcement members, the present invention specifies that the extruded member has the texture in which intergranular precipitates 1 μm or larger in terms of the diameter of an equivalent circle are 3 μm or more separate from one another on average in the observation under a TEM of 5000 magnifications. The average distance between intergranular precipitates should preferably be 5 μm or larger, more preferably be larger than 10 μm.

The term “intergranular precipitates” used in the present invention denotes such compounds as MgSi or Si in the form of simple substance, which are expected from the composition of the aluminum alloy of 6000 series. MgSi forms the β′ phase to increase the strength of the extruded member of aluminum alloy of 6000 series used as reinforcement members. Intergranular precipitates, however, are harmful if they are excessively coarse or they exist in an excessively large amount; they will start rupture and propagate rupture even though the texture is controlled as mentioned above, thereby deteriorating the flexural crushing performance and corrosion resistance of the extruded member used as automotive reinforcement members. The distance between intergranular precipitates is specified as above in order that the extruded member of aluminum alloy of 6000 series, which has the texture with well-developed cube orientation as mentioned above, exhibits good flexural crushing performance and corrosion resistance.

In the case where intergranular precipitates 1 μm or lager in terms of the diameter of an equivalent circle are less than 3 μm separate from one another on average in the observation under a TEM of 5000 magnifications, intergranular precipitates are coarse or excessively close to one another and are distributed densely. Therefore, they start intergranular rupture or corrosion and propagate them when they receive flexural loads at the time of collision. Therefore, the extruded member used as automotive reinforcement members decreases in flexural crushing performance and corrosion resistance even though the texture is controlled as mentioned above. Incidentally, intergranular precipitates smaller than 1 μm in terms of the diameter of an equivalent circle do not affect flexural crushing performance and corrosion resistance so much. Therefore, the size of intergranular precipitates is not specifically defined to avoid its confusion with the distance between intergranular precipitates.

(Measurement of Average Distance Between Intergranular Precipitates and Size of Intergranular Precipitates)

Measurement of the average distance between intergranular precipitates and the size of intergranular precipitates is performed on the equiaxed recrystallized grain structure in the cross section of the extruded member. Unlike the texture to be observed as mentioned above, this structure is at the center of the cross section in the thickness direction of the extruded member, with the GG layer in the outermost surface being excluded. The specimen for equiaxed structure is made into a thin film, which is subsequently observed under a TEM of 5000 magnifications.

Observation under a TEM is designed to examine the structure in a much smaller (micro) region than the crystal orientation analysis by means of SEM/EBSP mentioned above. It needs a huge number of measurements over the entire region in the thickness direction of the extruded member. Therefore, in actual practice, observation is performed at one point in the center of the thickness such that the total field of view is 40 μm² or larger, and this procedure is repeated at ten points an adequate distance apart in the lengthwise direction of the extruded member and the resulting data are averaged. The size of each intergranular precipitate is expressed in terms of the diameter of an equivalent circle. All the intergranular precipitates in the field of view are examined for the diameter of equivalent circle, and those which are 1 μm or larger are selected. The average distance between the adjacent intergranular precipitates thus selected are measured and the resulting data are averaged.

(Chemical Composition)

According to the present invention, the extruded member of aluminum alloy of 6000 series has the chemical composition as follows. It needs good flexural crushing performance and corrosion resistance so that it is used as automotive reinforcement members as mentioned above.

To meet this requirement, the extruded member of aluminum alloy of 6000 series covered by the present invention (or the cast billet as a raw material thereof) should be an Al—Mg—Si aluminum alloy containing (in mass %,) Mg: 0.60-1.20%, Si: 0.30-0.95%, Fe: 0.01-0.40%, Mn: 0.001-0.35%, Cu: 0.001-0.65%, Zn: 0.001-0.25%, and Ti: 0.001-0.10%, with the remainder being aluminum and inevitable impurities. It may further selectively contain at least either of Cr: 0.001-0.18% or Zr: 0.001-0.18% in a total amount of 0.30% or less. Percentage (%) for the content of each element is in terms of mass %.

Any other elements than listed above are basically impurities. The content of such impurities should be lower than the level allowed by the AA and JIS standards. However, contamination with impurities is liable to occur when the melt is prepared from not only high-purity aluminum ground metal but also scraps of 6000-series alloy and other aluminum alloys in large amounts for the purpose of recycling. Reducing these impurity elements below the detection limit increases production cost, and a certain level of their content should be allowed. Therefore, other elements than listed above may be allowed according to the AA and JIS standards.

The following is the base on which the content of each element listed above is established for the aluminum alloy of 6000 series.

Si:

The content of Si should be 0.30-0.95%, which depends on the content of Mg. The preferred content of Si should be 0.30-0.50% to give the balance alloy mentioned above. Both Si and Mg are essential elements which cause solid solution strengthening and forms age precipitates (which contribute to strengthening) in crystal grains at the time of artificial aging treatment, thereby exhibiting the ability of age strengthening and producing strength (proof stress) of 200 MPa or greater necessary for reinforcement members. With too small a content, Si does not form the above-mentioned compound phase at the time of artificial aging treatment, with the age hardening and desired strength not attained. With an excess content, Si does not give the balance alloy which has the texture specified in the present invention. An excessively low content of Si is detrimental to bending and weldability.

Mg:

The content of Mg should be 0.60-1.20%, which depends on the content of Si. The preferred content of Mg should be 0.61-1.0% to give the balance alloy mentioned above. Mg together with Si is an essential element which causes solid solution strengthening and forms age precipitates (which contribute to strengthening) in crystal grains at the time of artificial aging treatment, thereby exhibiting the ability of age strengthening and producing strength (proof stress) greater than 200 MPa necessary for reinforcement members. With too small a content, Mg does not form the above-mentioned compound phase at the time of artificial aging treatment, with the age hardening and desired strength not attained. With an excess content, Mg does not give the balance alloy. An excessively low content of Mg is detrimental to bending.

Content of Mg and Si:

In order that the extruded member of aluminum alloy of 6000 series has the equiaxed recrystallized grain structure, in which the average areal ratio of cube orientation exceeds 15% and intergranular precipitates 1 μm or higher in terms of the diameter of an equivalent circle are no less than 3 μm apart on average, the content of Mg and Si should be such that Mg(%)≧1.73×Si(%)−0.4, preferably Mg(%)≧1.73×Si(%)−0.2. This relationship was established for the aluminum alloy of 6000 series specified in the present invention to be a balance alloy which contains Mg and Si in a stoichiometrically equivalent amount or an Si-excess aluminum alloy with a comparatively small content of Si.

An aluminum alloy of 6000 series which contains Si in an amount more than specified by Mg≧1.73×Si, or an aluminum alloy of Si-excess type which contains Si in a large excess amount, increases in proof stress due to artificial age hardening treatment at a comparatively low temperature and exhibits good age hardening performance (BH performance) that imparts necessary strength. Therefore, it is commonly used in the field of aluminum alloy of 6000 series which needs good formability and high strength after forming so that it is made into automotive panels by press forming or bending.

However, if the extruded member of aluminum alloy of 6000 series according to the present invention has the Si-excess composition, Si remains unmelted during extrusion and becomes nuclei having various crystal orientations, resulting in the texture with random orientations, with the development of cube orientation suppressed and the ratio of cube orientation remarkably decreased. It also tends to have the above-mentioned fibrous structure elongating in the direction of extrusion.

For this reason, any extruded member produced from an aluminum alloy of 6000 series containing excess Si would not have the equiaxed recrystallized grain structure having cube orientations such that their average areal ratio is 15% or higher over the entire region in the thickness direction of the extruded member, said thickness including the grain growth layer in the outermost surface. (This depends on the manufacturing condition such as extrusion.) Moreover, an excess Si content gives rise to a large number of coarse intergranular precipitates arising from Si, which would prevent the formation of the above-mentioned texture and the structure in which intergranular precipitates 1 μm or lager in terms of the diameter of an equivalent circle are no less than 3 μm apart on average, which is necessary for the extruded member to exhibit good flexural crushing performance and corrosion resistance. Therefore, if the Si content exceeds an amount specified by Mg(%) ≧1.73×Si(%)−0.4, or more stringently Mg(%)≧1.73×Si(%)−0.2, the extruded member would not exhibit good flexural crushing performance and corrosion resistance when used as reinforcement members. (This depends on the manufacturing conditions such as extrusion.)

Fe:

Fe functions in the same way as Mn, Cr, and Zr to form dispersed particles (dispersion phase), hampers intergranular movement after recrystallization, prevents crystal grains from becoming coarse, and makes crystal grains fine. Fe is an element which inevitably originates in a certain amount (substantial amount) from scraps as a raw material for molten metal. The content of Fe should be 0.01-0.40%. Fe does not produce its effect if its content is excessively small. Fe in an excess content tends to give rise to coarse crystals such as Al—Fe—Si crystals, which deteriorate fracture toughness and fatigue characteristics.

Mn:

Mn is a transition metal element like Cr and Zr; it prevents crystal grains from becoming coarse. It selectively combines with other alloying elements to form dispersed particles (dispersion phase) of intermetallic compound such as Al—Mn at the time of soaking heat treatment and ensuing hot extrusion. These dispersed particles are fine and dispersed densely and uniformly (to varied degrees depending on the manufacturing conditions), so that they effectively hinder intergranular movement after recrystallization and prevent crystal grains from becoming coarse and make crystal grains fine. Mn in an excessively small amount does not produce these effects but makes crystal grains coarse (under certain manufacturing conditions) to cause the extruded member to decrease in strength and toughness. Mn also dissolves in the matrix to increase strength.

Excess Mn, however, causes the extruded member to have the fibrous structure elongating in the direction of extrusion. Thus it prevents the formation of the equiaxed recrystallized grain structure in which cube orientation has an average areal ratio larger than 15% over the entire region in the thickness direction of the extruded member. Moreover, excess Mn tends to form, at the time of melting and casting, coarse intermetallic compounds and crystals which start rupture and cause the extruded member (as reinforcement members) to decrease in flexural crushing performance, corrosion resistance, and bendability. Therefore, an adequate content of Mn should be 0.001 to 0.35%, and a minimal content is desirable.

Cu and Zn:

Cu and Zn contribute to strength through solid solution hardening and also remarkably promote age hardening while the final product is undergoing aging treatment. The content of Cu and Zn should be 0.001-0.65% and 0.001-0.25%, respectively. Cu and Zn in an excessively small content do not produce the effects mentioned above. On the other hand, excess Cu and Zn make the extruded member highly sensitive to stress corrosion cracking and intergranular corrosion, thereby deteriorating corrosion resistance and durability. If Cu and Zn are to be contained, their content should be as specified above.

Ti:

Ti makes crystal grains in an ingot fine and causes the extruded member to have the structure composed of fine crystal grains. The extruded member should be incorporated with Ti in an amount of 0.001-0.10%. If the source of Ti contains B. the content of B should be 1-300 ppm. Ti in an excessively small amount does not produce the above-mentioned effect. Excess Ti, however, forms coarse crystals and causes the extruded member (as reinforcement members) to decrease in flexural crushing performance, corrosion resistance, and bendability. Therefore, an adequate content of Ti should be in the range specified above.

At least either of Cr and Zr:

Cr and Zr, which are transition metal elements, form dispersed particles (dispersion phase) of intermetallic compound, such as Al—Cr and Al—Zr, thereby preventing crystal grains from becoming coarse, in the same way as Mn. However, excess Cr and Zr, like excess Mn, cause the extruded member to have the fibrous structure which elongates in the direction of extrusion. Therefore, the content of Cr should be 0.001-0.18% and the content of Zr should be 0.001-0.18%, and their total content should be no more than 0.30%. Their content should be as low as possible.

(Sectional Form of Extruded Member)

The extruded member of aluminum alloy of 6000 series should have a specific sectional form so that it exhibits good flexural crushing performance when used as reinforcement members. The sectional form should preferably be hollow so that the extruded member has light weight and good flexural crushing performance required of reinforcement members. The hollow sectional form should typically (basically) be rectangular. The rectangular form consists of two flanges (front and rear walls) and two webs (upper and lower walls connecting both flanges). The rectangular cross section may additionally have one or more inner ribs for reinforcement (or for improvement in flexural crushing performance). Possible arrangement of such inner ribs may be a single rib or double ribs parallel to the upper and lower side walls or cross ribs connected to four corners of the cross section.

The sectional form may be modified such that the flange is wider than the distance between the webs (or the edges of the flange extend beyond the webs) or the flange and web are curved inward or outward. The hollow sectional form may be uniform over the entire length of the extruded member or may vary from one place to another along the length. The extruded member to be used as the bumper reinforcement may have a hollow sectional form which is not completely closed but is partly opened. This sectional form is less strong than the completely closed one and disadvantageous for weight saving and flexural crushing performance.

(Wall Thickness of Extruded Member)

The extruded member should have an adequate wall thickness in relation to the sectional form so that it exhibits good flexural crushing performance required of reinforcement members. Since the present invention is intended for automotive reinforcement members that absorb energy at the time of collision, the extruded member should have a certain thickness unlike body panels of rolled thin sheet, so that it exhibits good flexural crushing performance required of reinforcement members. A greater thickness is desirable for good flexural crushing performance but an excessively great thickness increases weight, which is contrary to weight saving. Therefore, an adequate wall thickness should be selected from a range of 2-7 mm. It is not always necessary that the flanges, webs, and inner ribs constituting the above-mentioned sectional form have the same thickness, but they vary in thickness. For example, the flange which receives loads at the time of collision may be thicker than other parts.

(Manufacturing Method>

The following is a description of the method for producing the extruded member of aluminum alloy of 6000 series. The extruded member according to the present invention denotes one which undergoes refining, such as quenching and artificial aging treatment, after hot extrusion. The manufacturing process itself is ordinary and known, except for the conditions of controlling the texture. However, for the extruded member to have the texture with cube orientations within the range specified in the present invention, the manufacturing method should include the soaking step which is controlled at a specific cooling rate.

The manufacturing method for the extruded member according to the present invention starts with preparing a billet from the aluminum alloy of 6000 series. The billet undergoes soaking, which is followed by cooling approximately to room temperature. The billet is heated again to a temperature for solution treatment and then subjected to hot extrusion. The extrudate is immediately cooled approximately to room temperature by water cooling (for forced cooling) on-line. In this way there is obtained the extruded member having the specific sectional form mentioned above. The extruded member that has passed through a series of hot extrusion steps also has undergone solution and quenching treatment. Subsequently, the extruded member undergoes cutting and leveling treatment and optional refining such as artificial age hardening. Alternatively, the artificial age hardening may be performed simultaneously with paint baking after the extruded member (as a reinforcement member) has been built into the automotive body and the automotive body has been painted, instead of being performed preliminarily while the extruded member still remains as such.

Melting and Casting:

An aluminum alloy having the above-mentioned composition conforming to 6000 series is melted, and the molten metal is cast in the usual way, such as continuous casting and semicontinuous casting (DC casting).

(Soaking Heat Treatment)

The billet of aluminum alloy which has been cast as mentioned above subsequently undergoes soaking heat treatment. Soaking is performed in the usual way at a temperature of 500° C. or higher and lower than melting point, preferably at 500-590° C. Soaking is intended to homogenize the structure, or to eliminate segregation from the crystal grains in the structure of the billet, thereby making alloy elements and coarse compounds into a complete solid solution. Soaking at a lower temperature than specified above does not completely eliminate segregation from crystal grains; residual segregation starts rupture and deteriorates flexural crushing performance, mechanical properties, and bendability.

After soaking, the billet undergoes forced cooling to 400° C. (and down to room temperature) at an average cooling rate of 100° C./hr or above. Forced cooling should be accomplished at as high a cooling rate as possible by air blowing or with water. Once 400° C. is reached, forced cooling is continued or switched to self-cooling down to room temperature.

The cooling rate mentioned above is quite different from the one employed in the case where ordinary billets are allowed to cool outside the soaking pit. In this case the cooling rate is usually about 40° C./hr at the highest, depending on the size of billets; it never exceeds 100° C./hr mentioned above. The result of such slow cooling is that MgSi compounds dissolve temporarily into solid solution during soaking treatment at a high temperature but combine with FeAl compounds, which remain undissolved because of their high melting point, during cooling, to form another composite compounds (precipitates). Such precipitates remain undissolved in the extrusion process and become nuclei having various crystal orientations like excess Si mentioned above, thereby altering the structure into the texture with random orientations. This prevents the development of cube orientations and remarkably decreases the ratio of cube orientations.

The billet undergoes reheating and hot extrusion in such a way that the temperature of the extrudate (at the exit of the extruder) is 500° C. or above, which is high enough to keep the extrudate in solution form. Immediately after extrusion, the extrudate undergoes forced cooling at an average cooling rate of 100° C./min or higher. This forced cooling is necessary to achieve T5 refining, which may be combined with T6 refining (aging) or T7 refining (over aging). For T5 refining, the extrudate at the exit of the extruder is kept at 500° C. or above, which is high enough to keep the extrudate in solution form. The extrudate undergoes solution treatment on-line (as the result of extrusion) and, immediately thereafter, undergoes forced cooling (for quenching) down to the neighborhood of room temperature on-line.

The temperature at the time of hot extrusion should be rather low so that cube orientations develop easily and the texture of the extruded member becomes the equiaxed recrystallized grain structure in which the average areal ratio of cube orientations is no less than 15% over the entire region in the thickness direction of the extruded member. However, if the temperature of the extrudate at the exit of the extruder is lower than 500° C. (which is solution temperature), coarse Mg—Si compounds (precipitates) remain undissolved in the matrix and they start rupture to deteriorate flexural crushing performance and corrosion resistance. To meet the contradictory requirements, it is desirable to select a lowest possible temperature of 500° C. or above for the extrudate at the exit of the extruder. However, it is not always necessary to reheat the billet of 500° C. or above for extrusion, because even though the reheating temperature is below 500° C., the temperature of the extrudate is 500° C. or higher on account of heat generation by hot extrusion.

Forced cooling for quenching with water that immediately follows extrusion is intended for the extruded member as reinforcement members to improve in flexural crushing performance and corrosion resistance. Forced cooling alters the texture of the extruded member into the equiaxed recrystallized grain structure in which the average areal ratio of cube orientations is 15% or higher over the entire region in the thickness direction of the extruded member. In addition, forced cooling also gives rise to intergranular precipitates which are 1 μm or larger in terms of the diameter of an equivalent circle and are separated from one another at an average interval of 3 μm and above. Forced cooling that immediately follows extrusion should be accomplished on-line with any cooling means arranged near the exit of the extruder, such as shower for water mist or spray, water bath, and air blower, or a combination thereof. The cooling rate for forced cooling is 100° C./min or above, which is much higher than that (about 50° C./min) for the extrudate which is allowed to cool.

The T5 refining treatment omits post-extrusion steps such as reheating, solution treatment, and quenching for the extruded member. Under certain circumstances, the T5 refining treatment may be replaced by the T6 refining treatment which consists of separate reheating of the extruded member at 500° C. and above, and ensuing solution treatment, quenching, and artificial aging that follow extrusion.

Ageing Treatment:

The extruded member undergoes artificial aging treatment after cutting to length and leveling treatment. The artificial aging treatment should be carried out at 150-250° C. for a prescribed period of time. Duration of aging treatment controls age hardening; it should be properly selected to maximize strength or extended for averaging that improves corrosion resistance.

EXAMPLES

The examples of the present invention will be described below. Samples of extruded members were prepared from aluminum alloy of 6000 series varying in composition as shown in Table 1 and under different conditions as shown in Table 2. The extruded member has a rectangular sectional form with a center rib. Each sample was examined for structure and characteristics (such as mechanical properties, flexural crushing performance, and corrosion resistance). Each sample in Table 1, except for Comparative Example 5, contains Mg and Si in such an amount as to satisfy the following relation.

Mg(%)≧1.73×Si(%)−0.4, or Mg(%)≧1.73×Si(%)−0.2

To be concrete, each sample of the extruded member was prepared as follows. First, the aluminum alloy whose composition is shown in Table 1 was melted and cast into a billet. The billet underwent soaking treatment at a temperature shown in Table 2, and soaking was followed by cooling to room temperature at an average cooling rate (° C./hr) shown in Table 2. The average cooling rate was 120° C./hr in the case of forced cooling by a blower and 40° C./hr in the case of self-cooling. The cooled billet was heated again and immediately subjected to hot extrusion at an extrusion rate (m/min) and a temperature (° C.), measured at the exit of the extruder, which are shown in Table 2. Immediately after extrusion, the extrudate underwent forced cooling to the neighborhood of room temperature with the help of cooling means shown in Table 2. Thus there was obtained an extruded member having a square sectional form with a central rib. The forced cooling was carried out with water or air at a cooling rate of about 50° C./s or about 20° C./s, respectively. The resulting extruded member underwent artificial age hardening treatment for 3 hours at a temperature shown in Table 2.

The sectional form of the extruded member has the following dimensions. The flanges (or the front and rear walls) are 40 mm long and 2.3 mm thick. The webs (or the side walls) and the central rib are 40 mm long and 2.0 mm thick. The extruded member was cut to a length of 1300 mm.

After artificial age hardening treatment, the web of the extruded member was cut into a test sample in sheet form. The test sample was examined for structure and characteristic properties. The results are shown in Table 2.

(Structure of Test Sample)

Average Areal Ratio of Cube Orientation:

After refining as mentioned above and aging at room temperature for 15 days, the test sample was examined for texture by means of SEM-EBSP. The texture was analyzed to obtain the average areal ratio (%) of cube orientation over the entire region of the cross section in the thickness direction, including the grain growth layer in the outermost surface. If the average areal ratio (%) of cube orientation is subtracted from 100%, the remainder is the average total areal ratio of other orientations than cube orientation, which include Goss, CR, RW, Brass, S, Cu, and SB orientations.

Each test sample was also examined by SEM-EBSP for the recrystallized grain structure in terms of the aspect ratio of crystal grains. The structure in which crystal grains have an average aspect ratio smaller than 5 or greater than 5 was designated as the equiaxed granular structure or the fibrous structure, respectively.

Average Distance Between Intergranular Precipitates:

After refining as mentioned above and aging at room temperature for 30 days, the test sample was examined for the structure in the thickness direction by observation under a TEM of 5000 magnifications as mentioned above so as to measure the average distance (μm) between intergranular precipitates 1 μm or lager in terms of the diameter of an equivalent circle. The results are shown in FIG. 2.

(Characteristics of Test Sample)

After refining as mentioned above and aging at room temperature for 30 days, the test sample was examined for characteristic properties, such as 0.2% proof stress (As proof stress in MPa), elongation (%), flexural crushing performance, and corrosion resistance. The results are shown in Table 2.

Tensile Test:

The test sample was cut into a specimen for tensile test, No. 5 conforming to JIS Z2201, which measures 25 mm wide, 50 mm long, and 2.0 mm thick as extruded. The specimen length and tensile force are parallel to the direction of extrusion. The tensile force was applied at a rate of 5 mm/min up to 0.2% proof stress and 20 mm/min thereafter. Five measurements were averaged.

Test for Flexural Crushing Performance:

The test sample (in sheet form) was bent to 180° according to the press-bending method (JIS Z2248), in the direction perpendicular to the direction of extrusion. The bending test was repeated, with the bending radius (R mm) gradually reduced to the limit at which cracking occurs in the outside of the bent corner (or in the stretched side). Any test sample having the critical bending radius greater than 3.0 mm is regarded as good in flexural crushing performance and suitable for use as automotive reinforcement members.

Corrosion Resistance:

The test sample mentioned above was tested for corrosion resistance by dipping under the following conditions according to ISO/DIS 11846B. The test method consists of dipping the sample in an aqueous solution containing 30 g/L of NaCl and 10 mL/L of HCl for 24 hours at room temperature and subsequently observing the cross section of the sample to see intergranular corrosion cracking. The sample is rated by the following criterion.

-   ×: Intergranular corrosion cracking occurred. -   Δ: Intergranular corrosion occurred but intergranular corrosion     cracking did not occur. -   ◯: Neither intergranular corrosion cracking nor intergranular     corrosion occurred (even though corrosion occurred all over the     surface).

As shown in Tables 1 and 2, samples in Examples 1 to 18 contain Mg and Si in an amount specified by the present invention and undergo soaking and hot extrusion under preferred conditions with regard to soaking temperature, forced cooling that follows soaking, temperature at the extruder exit, extrusion speed, and forced cooling with water that follows immediately after extrusion. Therefore, they have the equiaxed recrystallized grain structure having the cube orientation and the average intervals of intergranular precipitates, as specified in the present invention, and hence they excel in flexural crushing performance and corrosion resistance, and they also excel in mechanical properties such as strength and elongation. These outstanding characteristic properties suggest that the extruded member is suitable for use as automotive reinforcement members which might encounter more serious collisions such as pole collision and offset collision, and that the extruded member has good flexural crushing performance and good corrosion resistance required of reinforcement members.

By contrast, the samples in Comparative Examples 1 to 4 have the composition (shown in Table 1) conforming to the present invention but they are produced under conditions not conforming to the present invention. Therefore, as shown in Table 2, they do not have the specific equiaxed recrystallized grain structure having cube orientations and/or the specific average intervals for intergranular precipitates, as specified in the present invention. Thus, the samples in these Comparative Examples are inferior in flexural crushing performance and/or corrosion resistance to those in Working Examples.

Comparative Example 1 shows the effect of an excessively low cooling rate employed after soaking. Comparative Example 2 shows the effect of an excessively low cooling rate (due to air cooling) employed after immediately after extrusion. Comparative Example 3 shows the effect of an excessively low soaking temperature. Comparative Example 4 shows the effect that is produced when the temperature at the exit of the extruder is excessively lower than the solid solution temperature.

The samples in Comparative Examples 5 to 13 are produced under the desirable conditions shown in Table 2 but have the composition shown in Table 1 which is outside the range specified in the present invention. Therefore, as shown in Table 2, they do not have the specific equiaxed recrystallized grain structure having cube orientations and/or the specific average intervals for intergranular precipitates, as specified in the present invention. Thus, the samples in these Comparative Examples are inferior in flexural crushing performance and/or corrosion resistance to those in Working Examples.

The sample in Comparative Example 5 contains too much Si, and hence the content of Mg and Si therein does not satisfy the following relation.

Mg(%)≧1.73×Si(%)−0.4, or Mg(%)≧1.73×Si(%)−0.2

The sample in Comparative Example 6 contains too much Mg. The sample in Comparative Example 7 contains too much Cu. The sample in Comparative Example 8 contains too much Mn. The sample in Comparative Example 9 contains too much Zr. The sample in Comparative Example 10 contains too less Fe. The sample in Comparative Example 11 contains too less Si. The sample in Comparative Example 12 contains too much Zn. The sample in Comparative Example 13 contains too much Ti.

The foregoing results of Examples demonstrate that the composition, structure, and manufacturing conditions specified in the present invention are essential for the extruded member to have good flexural crushing performance, corrosion resistance, and mechanical properties.

TABLE 1 Composition of Al alloy (in mass %, in ppm for B, remainder is Al) Division Number Si Fe Cu Mn Mg Cr Zn Ti Zr Working 1 0.420 0.200 0.140 0.020 0.800 0.050 0.002 0.020 0.000 Examples 2 0.370 0.200 0.150 0.180 0.800 0.000 0.010 0.020 0.000 3 0.420 0.200 0.150 0.002 0.800 0.100 0.003 0.020 0.000 4 0.420 0.210 0.130 0.200 0.800 0.050 0.003 0.020 0.000 5 0.420 0.200 0.150 0.003 0.800 0.000 0.003 0.020 0.080 6 0.420 0.200 0.020 0.020 0.800 0.050 0.010 0.020 0.020 7 0.530 0.210 0.140 0.200 0.910 0.050 0.020 0.020 0.000 8 0.510 0.190 0.140 0.200 0.790 0.050 0.010 0.020 0.000 9 0.490 0.190 0.140 0.200 0.780 0.000 0.020 0.020 0.000 10 0.550 0.210 0.350 0.200 0.900 0.050 0.005 0.020 0.000 11 0.570 0.200 0.510 0.330 0.950 0.050 0.000 0.020 0.000 12 0.420 0.200 0.530 0.003 0.800 0.050 0.003 0.020 0.000 13 0.420 0.200 0.150 0.002 0.800 0.050 0.050 0.020 0.000 14 0.420 0.200 0.150 0.003 0.800 0.050 0.150 0.020 0.000 15 0.420 0.200 0.150 0.020 0.800 0.050 0.003 0.050 0.000 16 0.600 0.200 0.150 0.100 1.000 0.050 0.010 0.020 0.000 17 0.700 0.150 0.150 0.100 1.100 0.000 0.010 0.020 0.020 18 0.380 0.050 0.150 0.220 0.800 0.100 0.020 0.020 0.010 Comparative 1 0.420 0.200 0.140 0.020 0.800 0.050 0.000 0.020 0.000 examples 2 0.400 0.200 0.140 0.020 0.800 0.050 0.000 0.020 0.000 3 0.700 0.200 0.100 0.020 1.150 0.050 0.000 0.000 0.000 4 0.700 0.200 0.100 0.020 1.150 0.050 0.000 0.000 0.000 5 1.000 0.200 0.150 0.050 1.000 0.050 0.000 0.020 0.000 6 0.850 0.200 0.150 0.050 1.300 0.050 0.010 0.020 0.000 7 0.400 0.200 0.800 0.050 0.800 0.050 0.010 0.020 0.000 8 0.400 0.200 0.150 0.370 0.800 0.050 0.020 0.020 0.000 9 0.400 0.200 0.150 0.150 0.800 0.200 0.000 0.020 0.200 10 0.400 0.500 0.150 0.100 0.800 0.100 0.000 0.020 0.000 11 0.220 0.200 0.150 0.100 0.400 0.050 0.000 0.020 0.000 12 0.400 0.200 0.150 0.050 0.800 0.050 0.350 0.020 0.000 13 0.400 0.200 0.150 0.050 0.800 0.050 0.000 0.200 0.000

TABLE 2 (Continued from Table 1) Conditions of extrusion Conditions of soaking Cooling treatment means Rate of cooling from applied soaking temperature Extrusion immediately Aging Soaking temperature to room temperature Extrusion temperature speed after treatment Division Number (° C.) (° C./hr) at exit (° C.) (m/min) extrusion ° C. × 3 hr Working 1 520 120 530 4 Water spray 190 Examples 2 580 120 530 3 Water spray 190 3 550 120 530 10 Water spray 190 4 580 120 530 3 Water spray 190 5 550 120 530 3 Water spray 190 6 550 120 530 3 Water spray 190 7 550 120 550 10 Water spray 190 8 550 120 550 10 Water spray 190 9 550 120 550 10 Water spray 190 10 550 120 550 10 Water spray 190 11 550 120 550 10 Water spray 190 12 550 120 530 3 Water spray 190 13 550 120 530 3 Water spray 190 14 500 120 530 3 Water spray 190 15 550 120 530 3 Water spray 190 16 580 120 530 10 Water spray 190 17 580 120 530 3 Water spray 190 18 580 120 530 3 Water spray 190 Comparative 1 550 40 530 3 Water spray 190 examples 2 550 120 530 3 Air blow 190 3 480 120 530 3 Water spray 190 4 550 120 480 3 Water spray 190 5 550 120 530 3 Water spray 190 6 550 120 530 3 Water spray 190 7 550 120 530 3 Water spray 190 8 550 120 530 3 Water spray 190 9 550 120 530 3 Water spray 190 10 550 120 530 3 Water spray 190 11 550 120 530 3 Water spray 190 12 550 120 530 3 Water spray 190 13 550 120 530 3 Water spray 190 Characteristics of extruded member Structure of extruded member Flexural Average crushing Average intervals performance Corrosion areal ratio between Critical resistance of cube intergranular Tensile 0.20% bending Intergranular Recrystallized orientations precipitates strength proof stress Elongation radius R corrosion Division Number grain structure (%) (μm) (MPa) (MPa) (%) (mm) susceptibility Working 1 Equiaxed grain 25 >30 277 252 12 0.5 ◯ Examples 2 Equiaxed grain 30 25 280 260 13 0.5 ◯ 3 Equiaxed grain 28 28 270 248 14 0.5 ◯ 4 Equiaxed grain 32 25 275 255 13 1.0 ◯ 5 Equiaxed grain 30 >30 265 242 15 0.5 ◯ 6 Equiaxed grain 18 >30 260 242 14 0.5 ◯ 7 Equiaxed grain 25 25 302 271 12 2.0 ◯ 8 Equiaxed grain 31 27 288 261 12 1.0 ◯ 9 Equiaxed grain 30 25 291 266 12 2.0 ◯ 10 Equiaxed grain 25 20 320 295 12 2.0 ◯ 11 Equiaxed grain 28 10 355 325 12 3.0 ◯ 12 Equiaxed grain 30 5 323 285 14 2.0 ◯ 13 Equiaxed grain 24 >30 275 253 13 0.5 ◯ 14 Equiaxed grain 28 20 285 265 11 2.0 ◯ 15 Equiaxed grain 24 >30 275 254 11 1.0 ◯ 16 Equiaxed grain 30 18 320 295 12 2.0 ◯ 17 Equiaxed grain 30 15 335 300 11 3.0 ◯ 18 Equiaxed grain 32 20 285 260 13 1.0 ◯ Comparative 1 Equiaxed grain 12 10 260 235 10 3.5 ◯ examples 2 Equiaxed grain 32 2.8 278 199 15 4.0 □ 3 Equiaxed grain 8 2 275 260 8 5.0 ◯ 4 Equiaxed grain 8 2 265 235 9 6.0 ◯ 5 Equiaxed grain 10 2.8 360 320 10 10.0 ◯ 6 Equiaxed grain 8 2.8 350 325 11 10.0 ◯ 7 Equiaxed grain 35 2 330 300 14 4.0 X 8 Fibrous grain 7 2.8 275 245 10 4.0 ◯ 9 Fibrous grain 6 2.8 265 240 11 4.0 ◯ 10 Equiaxed grain 8 2.8 255 230 8 10.0 ◯ 11 Equiaxed grain 25 >30 180 150 15 1.0 ◯ 12 Equiaxed grain 32 2 295 275 10 6.0 X 13 Equiaxed grain 10 >30 245 235 10 10.0 ◯

The present invention provides the extruded member of aluminum alloy of 6000 series and the manufacturing method therefor, said extruded member having both good flexural crushing performance and good corrosion resistance which are required of reinforcement members for automotive bodies. The extruded member is suitable for use as automotive body reinforcement members, such as bumper reinforcement and door guard bar, which need outstanding lateral crushing performance. 

1. An extruded member of aluminum alloy which contains (in mass %) Mg: 0.60-1.20%, Si: 0.30-0.95%, Fe: 0.01-0.40%, Mn: 0.001-0.35%, Cu: 0.001-0.65%, Zn: 0.001-0.25%, and Ti: 0.001-0.10%, with the remainder being aluminum and inevitable impurities, and has the metallographic structure whose cross section perpendicular to the direction of extrusion shows the equiaxed recrystallized grain structure in which intergranular precipitates 1 μm or lager in terms of the diameter of an equivalent circle are 3 μm or more separate from one another in the observation under a TEM of 5000 magnifications and also the average areal ratio of cube orientation is 15% or larger over the entire thickness region including the grain growth layer in the outermost surface in the cross section perpendicular to the direction of extrusion.
 2. The extruded member of aluminum alloy as defined in claim 1, which contains Mg and Si such that Mg(%)≧1.73×Si(%)−0.4. where Mg(%) and Si(%) denote the content of Mg and Si in mass %, respectively.
 3. The extruded member of aluminum alloy as defined in claim 1, which has the equiaxed recrystallized grain structure such that the average areal ratio of cube orientation is 20% or larger.
 4. The extruded member of aluminum alloy as defined in claim 1, which contains at least either of Cr: 0.001-0.18% or Zr: 0.001-0.18% in a total amount of 0.30% or less.
 5. The extruded member of aluminum alloy as defined in claim 1, which has flexural crushing performance such that the critical bending radius (R) is 3.0 mm or smaller which does not cause cracking in the 180° bending test according to JIS Z2248 in which the platy specimen is bent in the direction of extrusion, and which has corrosion resistance such that the specimen does not suffer intergranular corrosion in the alternating immersion corrosion test according to ISO/DIS 11846B.
 6. An energy absorbing member formed from the extruded member of aluminum alloy defined in claim 1, which crushes under load in the direction perpendicular to the direction of extrusion.
 7. A method for producing an extruded member of aluminum alloy, said method comprising a step of soaking a cast billet of aluminum alloy at 500-590° C., said billet containing (in mass %) Mg: 0.60-1.20%, Si: 0.30-0.95%, Fe: 0.01-0.40%, Mn: 0.001-0.35%, Cu: 0.001-0.65%, Zn: 0.001-0.25%, and Ti: 0.001-0.10%, with the remainder being aluminum and inevitable impurities, a step of subjecting the soaked billet to forced cooling to 400° C. or below at an average cooling rate of 100° C./hr or above, a step of reheating the cooled billet and subjecting the reheated billet to hot extrusion such that the extrudate reaches the solid solution temperature which is 500° C. or higher at the extruder exit, a step of immediately subjecting the extrudate to forced cooling at an average cooling rate of 100° C./hr or above, and a step of subjecting the cooled extrudate to aging, so that the resulting extruded member has a 0.2% proof stress of 240 MPa or greater and also has the metallographic structure whose cross section perpendicular to the direction of extrusion shows the equiaxed recrystallized grain structure in which intergranular precipitates 1 μm or lager in terms of diameter of an equivalent circle are 3 μm or more separate from one another in the observation under a TEM of 5000 magnifications and also the average areal ratio of the cube orientation is 15% or larger over the entire thickness region including the grain growth layer in the outermost surface in the cross section perpendicular to the direction of extrusion.
 8. The method for producing an extruded member of aluminum alloy as defined in claim 7, wherein the cast billet of said Al—Mg—Si aluminum alloy further contains at least either of Cr: 0.001-0.18% or Zr: 0.001-0.18% in a total amount of 0.30% or less. 